Bug 24903: Add tests for _handle_seqno

Bug 24903: Add tests for _handle_seqno

Trivial tests.

Test plan:
Run t/Serials/ModSerialStatus.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
(cherry picked from commit 71eb787fbd34294ff6df995fa74d930dec4f4057)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>